The declaration - long awaited by Orthodox leaders - could provide a new foundation for the pope's attempt to encourage dialogue between the two estranged branches of Christianity.
Especially important to Orthodox ears was clear remorse for the ``disastrous'' sacking of Constantinople by Crusaders in 1204 that contributed to the collapse of the Byzantine Empire about three centuries later. The fall of the city - now Istanbul, Turkey - is one of the festering disputes that have poisoned relations between the two churches.
``It is tragic that the assailants, which set out to secure free access for Christians to the Holy Land, turned against their own brothers in the faith. The fact that they were Latin Christians fills Catholics with deep regret,'' the pope said after arriving in Athens to begin a six-day pilgrimage to retrace the steps of the Apostle Paul. The trip also includes Syria and Malta.
The pontiff prayed for God's forgiveness for ``the occasions, past and present, when sons and daughters of the Catholic Church have sinned by action or omission against their Orthodox brothers and sisters.''
The head of the Greek Orthodox Church, Archbishop Christodoulos, clapped as the pope spoke.
``We are waiting for a bold word from your lips. ... And we have not heard even one word of apology until now,'' Christodoulos said moments before the pope's address.
The statement advances the pope's effort to begin the millennium with prayers of contrition for wrongs committed by Roman Catholics throughout the ages, including abuses against women and minorities. In March 2000 - on a similar biblical pilgrimage - the pope visited Israel's Holocaust memorial to say his church was ``deeply saddened'' by Christian persecution of Jews.
But the rift with the Orthodox may be one of the hardest to bridge.
Christianity split into the two branches nearly 1,000 years ago in disputes over papal authority. Some Orthodox clerics, led by Greeks and Russians, are highly critical of the Vatican and oppose any attempts at reconciliation.
The ill feelings draw from potent sources: religion, ethnic pride and a perception of historical injustices.
The ecumenical effort, however, would receive a major boost if supported by the Greek Orthodox, one of the pillars of faith for the world's more than 200 million Orthodox. Such a gesture by the Greek Orthodox leaders could unleash a huge internal crisis in the church.
``For something new to come about, something old must die,'' said Nicholas Constas, an assistant professor of theology at Harvard Divinity School.
No senior members of the Greek Orthodox Church were at the airport to greet the pope - underscoring the delicate and potentially tumultuous setting.
The 80-year-old pope, using a cane, was saluted by an Air Force honor guard. Two children from Greece's small Roman Catholic community held a bowl filled with Greek soil for the traditional papal kiss - a ceremony that had been in doubt over worries it could enrage Orthodox zealots opposing the visit.
Protesters - from monks to parish priests - plan more rallies during the pope's 24-hour stay. They rang church bells in a symbol of mourning and lowered to half-staff Greek flags and those of ancient Byzantium with its two-headed eagle.
Priests released black balloons with signs reading ``pope go home.''
Riot police blocked demonstrators from coming near Athens' Roman Catholic cathedral before the pope's scheduled appearance.
``The Vatican is the house of deception and criminal activity,'' shouted a Greek Orthodox cleric, Metropolitan Stephanos, through a bullhorn.
Some zealots had threatened to try to block the papal motorcade from reaching Areopagus hill, the judicial center of ancient Athens where Paul made his sermons in A.D. 51.
But large-scale opposition appeared to fizzle just hours before the pope's arrival. Some former protest leaders appealed for calm, apparently bowing to intense pressure from the government and mainstream church leaders.
Security forces, however, were taking no chances. More then 5,000 police officers fanned out across the city for patrols and roadblocks. Pedestrians were even stopped from strolling through central Athens, which had the feeling of a ghost town. The flag of Greece flew alongside the flag of the Holy See outside parliament.
``After 10 centuries,'' said the banner headline in the Eleftherotypia newspaper.